Thermodynamics and Chemistry

CHAPTER 7 PURE SUBSTANCES IN SINGLE PHASES

7.7 STANDARDSTATES OFPURESUBSTANCES 181

column of Table7.4.

7.7 Standard States of Pure Substances

It is often useful to refer to a reference pressure, thestandard pressure, denotedp. The
standard pressure has an arbitrary but constant value in any given application. Until 1982,
chemists used a standard pressure of 1 atm (1:01325 105 Pa). The IUPAC now recom-
mends the valuepD 1 bar (exactly 105 Pa). This book uses the latter value unless stated
otherwise. (Note that there is no defined standardtemperature.)
A superscript degree symbol () denotes a standard quantity or standard-state condi-
tions. An alternative symbol for this purpose, used extensively outside the U.S., is a super-
script Plimsoll mark ( ).^8
Astandard stateof a pure substance is a particular reference state appropriate for the
kind of phase and is described by intensive variables. This book follows the recommenda-
tions of the IUPAC Green Book^9 for various standard states.

 The standard state of apure gasis the hypothetical state in which the gas is at pressure

pand the temperature of interest, and the gas behaves as an ideal gas. The molar

volume of a gas at 1 bar may have a measurable deviation from the molar volume

predicted by the ideal gas equation due to intermolecular forces. We must imagine

the standard state in this case to consist of the gas with the intermolecular forces

magically “turned off” and the molar volume adjusted to the ideal-gas valueRT=p.

 The standard state of apure liquid or solidis the unstressed liquid or solid at pres-

surepand the temperature of interest. If the liquid or solid is stable under these

conditions, this is a real (not hypothetical) state.

Section9.7will introduce additional standard states for constituents of mixtures.

7.8 Chemical Potential and Fugacity

Thechemical potential,, of a pure substance has as one of its definitions (page 141 )

defDGmD

G

n

(7.8.1)

(pure substance)

That is,is equal to the molar Gibbs energy of the substance at a given temperature and
pressure. (Section9.2.6will introduce a more general definition of chemical potential that
applies also to a constituent of a mixture.) The chemical potential is an intensive state
function.
The total differential of the Gibbs energy of a fixed amount of a pure substance in a
single phase, withTandpas independent variables, is dGD SdTCVdp(Eq.5.4.4).

(^8) The Plimsoll mark is named after the British merchant Samuel Plimsoll, at whose instigation Parliament passed
an act in 1875 requiring the symbol to be placed on the hulls of cargo ships to indicate the maximum depth for
safe loading.
(^9) Ref. [ 36 ], p. 61–62.